1. Field of the Invention
The present invention relates to a method of and an installation for supplying air to a pulverized solid fuel burner.
The technical field of the invention is the manufacture of an industrial burner.
The main application of the invention is to the manufacture of burners for rotary furnaces supplied with fuel directly from the grinder producing the pulverized solid fuel by means of a pneumatic conveyor and without intermediate storage between said grinder and the burner.
2. Description of the Prior Art
Installations of the above kind in which all of the air passing through the grinder is used as conveyor air are well known in the art. The metering of the pulverized solid fuel is performed by weighing the unprocessed solid fuel which is then fed into the grinder. The grinder can also serve as a drier using hot air and/or smoke, whose maximum temperature can be as high as 400.degree. C. The combination of the hot drying air and the steam produced in this way then serves as a gas for conveying the pulverized solid fuel to the burner. The concentration of pulverized solid fuel relative to the conveyor air is relatively low, in the order of 0.3 kg to 0.6 kg per Nm.sup.3 of air under nominal conditions. This value varies as a function of the burner load, because the pulverized solid fuel is metered with a constant gas flowrate. For a variation of 1 to 3 the minimum concentration is therefore divided by 3. This high proportion of transport air, which is then wet and cold, at a temperature from 60.degree. C. to 90.degree. C., and which constitutes a major part of the primary air supporting combustion of the fuel in the burner, limits the efficiency of the rotary furnace and rules out adjustment of the shape of the flame.
A variant of what might be called direct combustion as described above has therefore been developed and is based on the same general principle, i.e. the fuel, such as coal, is metered before it is ground, and there is no intermediate storage of the fuel between the grinder and the burner; however, to enable the flowrate of air injected into the furnace to be reduced, an air/coal cyclone separator is installed on the conveyor line after the grinder and all of the air charged with fuel from the grinder is injected into this separator: this variant is referred to as semi-direct combustion. At the exit from the cyclone the pulverized solid fuel, referred to as PSF in the remainder of the description, is taken up by a pneumatic conveyor using purified air recovered from the outlet of the separator. The flowrate of air injected into the furnace in this way, transporting the PSF and forming primary combustion-supporting air, is lower than in the case of the direct combustion previously mentioned, because a portion of the drying gases is recycled after purification: this portion, corresponding to the portion of the purified air that is not used as the conveyor gas, is re-injected into the air supply circuit of the dryer-grinder. However, the quantity of primary air injected into the furnace remains high and the PSF concentration, which is undoubtedly higher than that obtained with direct combustion, remains limited to 0.6 kg/Nm.sup.3 to 1 kg/Nm.sup.3.
Accordingly, these old direct and semi-direct combustion technologies do not allow the use of modern burners like that described in European patent No. 0 421 903 filed Apr. 10, 1991 and used in ROTAFLAM type burners: in these burners the shape of the flame is adjusted using pulses generated at a high rate from two primary air circuits additional to that conveying the gas fuel, the total primary air used representing only a small percentage, around 6%, of the total combustion-supporting air. To minimize the effect of the conveyor air it is necessary to use a conveyor circuit with a high concentration, in the order of 3 kg/Nm.sup.3 to 6 kg/Nm.sup.3, i.e. around ten times greater than with direct combustion.
To this end, installations which might be referred to as indirect combustion installations have been constructed in which the PSF is stored between the grinder and the burner, which has the advantage of making the operation of the installation independent of the grinder. The PSF is metered at the exit from the storage silo by a continuous weighing system. The PSF is then conveyed by means of ambient air whose pressure can be increased at will and the minimized flowrate allows a relatively high PSF concentration of from 4 kg to 8 kg per Nm.sup.3, which represents an increase in the efficiency of the furnace. It is then possible to use multicircuit burners like those described in the aforementioned patent application with a low primary air rate which allows adjustment of the shape of the flame, improving heating and reducing the emission of pollutants such as nitrogen and carbon oxides. The major drawback of these indirect combustion schemes is their high cost, however; additional to this is the necessity to use a sleeve filter on the dust extraction circuit for the drying air because this air is discharged to atmosphere and can represent an explosion hazard for fuels with a high concentration of volatile materials.
The problem addressed is therefore that of providing installations for feeding air to a pulverized solid fuel burner enabling adjustment of the shape of the flame, for improved efficiency, and reduced formation of nitrogen oxides, in conformance with recent regulations, as in the case of the burners described in the previously mentioned patent EP 0 421 903, in particular by increasing the concentration of pulverized solid fuel relative to conveyor air to at least 2 kg per/Nm.sup.3 before it is injected into the burner. Such air and fuel supply installations must also not reject conveyor air that has also been used for drying, even after purifying it, because it still contains polluting and inflammable solid materials in suspension. Such installations must also be suitable for retrofitting to existing installations without major investment, such as adding heavy plant, as in indirect combustion installations.